This project contemplates high resolution light microscopic and electron microscopic studies on products of non-physiological fertilization and parthenogenetic activation of mammalian ova. Fertilization of aged mouse ova and consequent embryo development, spontaneous activation of follicular oocytes and tubal ova of mice of the LT strain, and artificial activation of mouse and rabbit ova by electric stimulation and heat and cold shock will be the main subjects of the proposed study. The aim of our investigation is to evaluate the most important differences which distinguish fertilization of aging ova and parthenogenesis from normal fertilization and embryo development.